The known ion-containing polymers include the sulfonated polystyrenes, copolymers of ethylene with alpha-beta unsaturated carboxylic acids such as acrylic acid or methacrylic acid and the fluorocarbon ionomers. The substantially fluorinated, fluorocarbon ionomers include those having pendant chains which contain sulfur-based functional groups, phosphorus-based functional groups and carboxylic acid or carboxylate functionality. All of these materials, with the exception of the phosphorus-based fluorocarbon ionomers, are presently commercially-available.
The substantially fluorinated ionomers which have pendant chains containing sulfonic acid functional groups or a salt thereof have been of particular interest, and commercial examples of such ionomers have been produced in the acid form by E. I. DuPont de Nemours & Co., Inc., under the Nafion.TM. trademark, where n is 1, 2, 3 etc. and the ratio of a:b is typically about 7 to 1: ##STR1##
The Dow Chemical Company has produced perfluorinated ionomers having a shorter side-chain (acid-form) structure, wherein n is 0 in the preceding formula: ##STR2##
The production of these perfluorinated ionomers is described widely in the literature, for example, in U.S. Pat. Nos. 3,282,875, 4,329,435, 4,330,654, 4,358,545 and 4,940,525, and is well known to those familiar with the perfluorinated ionomer art. Fundamentally, however, as related in U.S. Pat. No. 4,038,213, for example (referencing U.S. Pat. Nos. 3,282,875 and 3,882,093), both types of these perfluorinated ionomers can be typically prepared by the emulsion copolymerization of tetrafluoroethylene and fluorovinyl ethers that contain sulfonyl groups, and the subsequent transformation of the resulting sulfonyl fluoride precursor to the acid or salt form ionomer as desired.
As related in commonly-assigned, copending U.S. Ser. No. 08/404,476, now abandoned and U.S. Ser. No. 08/404,480, now abandoned, coatings have been applied from dispersions of these perfluorocarbon ionomers by evaporative coating techniques on various substrates, but the coatings produced by these processes have been less than satisfactory in one or more respects.
A significant focus of much of the literature to date has been the coating of polytetrafluoroethylene (PTFE) fibers and/or powders or particulate materials to make the PTFE fibers and/or particulate materials water-wettable. In this regard, PTFE possesses a number of desirable attributes, including excellent chemical stability. A significant barrier has existed however to the use of PTFE in certain applications, for example in the development of non-asbestos diaphragms for chlor-alkali cells, due to the hydrophobic nature of PTFE.
Various efforts have accordingly been made to compensate for or to overcome the hydrophobic character of PTFE in chlor-alkali diaphragms through the incorporation of ion-exchange materials by coating as well as by other means. An example of these efforts may be found in U.S. Pat. No. 4,169,024 to Fang, wherein PTFE (or a similar fluoropolymer) in the form of a powder or fibers, in an unsupported porous or nonporous film, in a coating on an inert fabric or in a porous reinforced structure (that is, a diaphragm) is chemically modified by reaction with a sulfur- or phosphorus-containing compound.
U.S. Pat. No. 4,720,334 to DuBois et al. is also representative, and describes diaphragms containing from 65 to 99 percent by weight of a fibrillated fluorocarbon polymer such as PTFE and from 1 to 35 percent of fluorocarbon ionomer (preferably containing carboxylic acid, sulfonic acid, alkali metal carboxylate or alkali metal sulfonate functionality) based on the combined weight of fibrillated fluoropolymer and ionomer, and optionally further containing wettable inorganic particulate material. The diaphragm is dried and secured upon an underlying cathode by being heated to a temperature below the sintering temperature of PTFE for a time.
The ionomer can be incorporated in the diaphragm of the DuBois patent by codeposition from a slurry with the ionomer being included as a solid, gel or solution, by being coated on either or both of the fluorocarbon fibrils and inorganic particulate and then deposited from a slurry, or by being extruded in admixture with the fluoropolymer before it is fibrillated. Specific coating processes for coating the PTFE fibrils are described, including mixing PTFE powder with a solution of ionomer in a water-miscible solvent under high shear conditions, then dispersing the coated fibrils by blending with water and some surfactant. Thereafter the materials are deposited onto the cathode from the resulting slurry.
Again, however, these efforts have not proven entirely successful. The aforementioned Ser. No. 08/404,476 and Ser. No. 08/404,480 applications (hereafter referred to as the '476 and '480 applications) accordingly offer alternative, improved processes for imparting improved wettability to PTFE to be incorporated into a chlor-alkali diaphragm, and especially a non-asbestos chlor-alkali diaphragm, via one or more thin, durable coatings of the relatively more costly perfluorocarbon ionomer applied thereto.
In the '476 application, for example, a colloidal, surface active dispersion of a perfluorosulfonate ionomer is used to contact a PTFE substrate, after which the coated PTFE substrate, while still wetted with the dispersion and without an intervening drying step, is exposed to a salt solution or to a solution of a strongly ionizing acid. Annealing of the coated PTFE, as in the bonding at from about 330 degrees Celsius to about 355 degrees Celsius of a diaphragm incorporating coated PTFE materials therein, results in a more durable, strongly adhering coating of the ionomer on the PTFE, but can result also in a reduced improvement in wettability as compared to an uncoated PTFE substrate.
The '480 application in turn is particularly directed to a solventless, batch coating process for coating PTFE, for example, with an ionomer coating, whereby the flammability and safety hazards associated with the use of a lower alcohol in making these sorts of dispersions can be avoided as well as the necessity of a step for evaporating the organic solvent, and in which PTFE is combined with a colloidal surface active dispersion of a perfluorocarbon ionomer in water and with a salt or strongly ionizing acid and the mixture is subjected to high shear conditions. Again, annealing conditions associated with the bonding of a non-asbestos diaphragm incorporating the coated PTFE provides improved durability and adhesion of the coating to the PTFE substrate, with however some reduction in wettability being seen generally in comparison to an unannealed, coated PTFE.